1. Field of the Invention
The present invention relates to an extruder suitably used for devolatilizing a material, such as plastic and rubber, in order to efficiently remove volatile components contained in the material.
2. Description of the Related Art
Generally, a composite resin material, such as a plastic compound, is manufactured by injecting a polymeric resin material and a powdery additive into a barrel of a continuous kneading unit, and then extruding the two downstream while they are kneaded with a kneading screw extending through the barrel. Furthermore, a granulator, for example, is disposed adjacent to the downstream side of the continuous kneading unit, such that the granulator forms the composite resin material into pellets.
A typical continuous kneading unit may either be a single kneading-screw type or a twin kneading-screw type.
A twin-screw continuous kneading unit includes a barrel having therein a pair of left and right kneading chambers which communicate with each other; a pair of left and right kneading screws extending through the two respective kneading chambers in a rotatable fashion; and supply means connected to an upstream portion of the barrel for injecting a material into the kneading chambers.
Each of the kneading screws includes screw segments for transferring the material downstream, and rotor segments and kneading segments that knead the material while applying a large shearing force to the material. These segments are connected to each other in an axial direction of the kneading screw.
For example, a twin-screw continuous kneading unit disclosed in Japanese Unexamined Patent Application Publication No. 2002-210731 has the structure described above, and is provided with a vacuum suction hole (vacuum vent) for forcibly removing volatile constituents and water from a material for the purpose of component adjustment. A gas suction operation through the vacuum vent creates a negative-pressure state in the kneading chambers, thereby enhancing devolatilization of the material and forcing the volatile components to be emitted outward through the vacuum vent.
Twin-screw continuous kneading units that are equipped with a material-extruding function are generally called twin-screw extruders.
According to Japanese Unexamined Patent Application Publication No. 2002-210731, however, the negative-pressure state generated in the kneading chambers in response to the gas suction through the vacuum vent extends only within a region in which the screw segments for transferring the material to be kneaded downstream are provided. This is problematic in that there are cases where the material is not sufficiently devolatilized.